Non-Orthogonal Multi-band CAP for Highly Spectrally Efficient VLC Systems

Non-Orthogona l Multi-band CAP for Highly Spectrally Eff icient VLC S ystems Paul Anthony Haigh 1* , Petr Chvojka 2* , Z. Ghassemlooy 3 , Stanislav Zvanove c 2 and Iz zat Dar wazeh 1 1 Comm unications and Info rmation Sys tems Grou p, Universi ty Co llege London , Lond on, WC1 E 6BT , UK 2 Departmen t of Electromagn etic Fi eld, Cz ech Techni cal Un iversity in Pragu e , Techni cka 2 , 16627, Pr ague, C zech Repu b lic 3 Optical Comm unications Resear ch Group, Northum bria Uni versity, Newcast le-upon- Tyne, NE1 8S T, UK {p.haigh; i.darwazeh} @ucl.a c.uk, { chvojpe8 ; xzv anove}@ fel.cvut .cz , z. gh assemloo y@northum bria .ac.uk * These au thors contributed equal ly Abstrac t — In this wo rk we propos e and e xp eri mentally demons trate a nove l n on-or thogo nal multi- ba nd carrier- le s s amplitud e and phase (N M - CAP) scheme for bandlimited vi sib le light co m munica tion sy st ems in o rde r to in creas e the spec tral effic iency . We s h ow that a bandw idth saving up to 30% can be achieved th us resulting i n 44 % improve ment i n t he m easu red s pectral e f ficiency with no fu rth er b i t error rate p erfor mance degradat i on comp ar ed to the tradi tiona l m - CAP sch eme . We al s o show t hat higher order s yste m s can prov i de hi gher bandwi dth compre s sion t han low order syste ms. Fur t hermo re, with no addition al function al blocks at the transm i tter or the receive r th e propo sed sche me introdu ces no ex t ra c omputa tional co m p lexity. Keywor ds— multi -band carrier -less amp lit u de and phase modula tion; no n-orthogon al; visible light com munic ations I. I NTRODUCTION In rec e nt y ears, v isib l e ligh t comm unicati ons (VLC ) has at tr a cted enorm ous attent ion within t he acade mic and indus t r ia l commun ities wor ld wi de [1] . V L C m ostly ut i li se s whit e light- emitt i ng dio de s (LEDs ) to suppor t sim ultaneo us dat a transm i ss ion and il lum inatio n in a hom e/offi ce environm e nt, and henc e, is a prom isin g sol uti on for the last -me ter con nectivi t y in futur e c omm unica tion networks (i.e. , 5 G ) [2] . Due t o t he limited modu lation ban dwidt h (i .e . a fe w M Hz) of the m ost comm only av ail ab le LED s [3 , 4 ] , the va st m ajor i ty of res earch ac tiviti e s have foc used on im plement ation of techn iques to demons trate the po tenti al of V LC links at h igh d ata r ates (i. e. from hundr eds of Mb /s to Gb / s) . The te chniq ues used ar e oft en bas e d on multi - carr i er m odul ation s chem e s in cludi ng or t hogo nal frequ ency divis ion m ultip l exi ng (OFDM) or c a rri er-less amp litud e and phase (CAP ) mo dulat ion [ 5, 6 ]. Generally, the rese a rch commun ity aims to achie ve high er t hrough put w i tho ut consid ering th e sp eci fic spe c tra l effi cienc ies of VLC sys tem s, which is a key perform ance metr ic r are ly r epor ted as part of t he link b it rate perform ances evalua tion. Recent ly, a numb er of methods hav e bee n prop osed to incre a s e spectral us age, su ch as fast OFDM (FOFDM ) as origi nally prop osed i n [7 ], spectr ally eff ici ent FDM (S EFDM ) [ 8] , dense OFDM (DO FDM) [9] or faste r than Nyqu ist (F T N ) sig nalli ng [ 10 ]. FOFDM mai nta ins th e sp ect ral ef f icien cy of an O FDM sys tem using Herm itian symm etry, albei t occup ying half the ban dwidth by redu cing the minim um frequ enc y s pac ing be t wee n subcarriers to ha lf th at of the symbo l r ate, i.e., 1/( 2 T ). Unf ortun ately , FOFDM is lim it ed only to one- dim e ns i ona l modu latio n s chemes (e. g. , bin ary pha se shift k e yin g (BPSK ) or puls e ampl i tud e m odula tion (P AM )) , henc e t he consta nt sp ectra l effici ency in comp arison t o OFD M . On th e o t her h and, SE F D M s upp orts tw o -d imensio nal modu lation sch e me s, wi th non-o rthog onal subcarr ier spa cing , which incr eas e s the s pectral e f fic ien c y at t h e cost o f self -indu ced in ter - carr ie r i nt er fe re nc e (I C I) be t w een subc arriers . In [8 ] i t wa s expe ri m e n tally demons trate d that up t o 25% bandw idth can be saved b y S EFDM in comp arison to OFDM wit h negl igi ble bit error rat e (BER) pe rform a nc e deg radatio n. Cont rary to OFD M- based t ech niques , th e F TN m etho d is impl emente d i n the time doma i n and oper ates on t he basis of non -orthogo nal fil t er d esig n . Howe ver, ak in to SEF DM , the imp rovem ent i n the ach iev e d spect ral ef ficien cy com es at t he cos t of the i ncreas ed tr ansm itter and rec eiver comp l exi t ies [10 ]. Multi -band CAP ( m -CAP) mod ulati on schem e has be en expe ri m e n tally dem ons tr ated as a p otent ial alter nat ive cand idate for h i gh ly bandlim ited V L C sys tems [11, 1 2] . Unli ke OFD M, m - CAP u tiliz es band -pass finit e impuls e r es pons e (F IR) filte rs to transm i t dat a streams in or thogon ally sp aced sub ca rrie rs as oppos ed to the i nve rse fast Fouri e r transf orm ( IFF T ). The F IR filt ers , w hich also ge ne ra t e th e c arri er f requ encies , ar e de ploy ed at both th e transmi tter and rec eiv e r . Th ese ar e cru ci al to m ainta i n th e req uired over a ll sys tem perform a nc e an d accoun t for mos t of the sys tem comp lex i ty as demons trated in [ 12 ]. By o p timi zing th e numb e r of fil ters used and their par ameters (i.e . the numb e r of taps and th e rol l - off fa ct or β , which de te r mi n es the exc ess bandw id th ) the req uired syst e m throu ghput and sp ectr a l effi c ien cy c an be impro ved [ 12] . In [ 13, 14 ], a n othe r appro ach for i m proving the V LC link capac i ty/s pectr a l effici ency w as intro duced by appl ying une qual l y spa ced sub carriers . The m os t recen t works hav e r epor ted tr ansm ission speed and sp ectr a l effi c ien cy up t o ~250 Mb/s [ 15] a nd ~6 b/s /Hz [ 12 ], respe ctively , thus pro vidi ng suff icient he a dr oom for fur ther impr ovement in V L C sys tems as repo rted based on n umeri cal simul ations in [ 16 ]. Hence , i n this work we propos e, for t he firs t t i me , and expe ri m e n tally v erif y a nov el non-o rthogon al sch e me te r m e d non-or thogo nal m - CAP ( NM - CA P ) . Th is sys tem adva ntageous ly improv es the spectral e fficien cy in VL C syst e ms. Th e pr opos ed nove l schem e is bas ed o n modif i ca t ion of the fil t e r ca rr ie r f re que nc ie s t o f or ce compr ession of su bcarri er s pac ing, be l ow ort hogon ality , and he nce c a use overl apping in th e adja cent f ilter pa ss bands . We an aly s e t he syst e m BER perform a nc e for a band width comp re ss ion up to 50% for a ra nge of excess bandw i d th param eter β = {0.1, 0 .2 , 0.3} and n umber of subc ar ri ers m = { 2, 10 }. Thes e valu es are chose n be cause the mi nim um numbe r of sub c arr iers th at ca n be used is 2 due t o the req uiremen t for mu ltipl e bands , wh ile m = 10 i s se lected b ecaus e the l ite ratu re has demo nstra ted t hat t he for sett i ng m > 10 , onl y limi ted im provem e nt in sp e ctr al eff i ci ency or data rate [15] is at tain ed . T he results s how that a bandw idth saving up to 30% ca n be achiev ed with no further d e gr ada tion in BER perfo rmanc e. Mor e ove r, t h ere is no ne e d for mod ifica tion of the re c ei ve r structu re and hen ce, no inc reas e in c om plexi t y i n comp a riso n to trad ition al m - CA P Fig. 2 Th e meas ure d LED an d n oise fre qu en cy res pons es with the h ig hlig hte d 3 dB level . Th e b an d - pass feat ure of t he LE D magni tude respo nse is caus e by the bias - tee ac tin g as a hig h - pa s s f ilte r and such, causi ng a cut - on e ff ec t a t aro und 2 50 kHz. The measured cut - off fr eq u e n cy i s a r ou n d 1. 2 5 M Hz , g iv i n g an effecti ve bandwidth o f aro u nd 1 MHz. I nset , the m easured I - V curve of the LED and drive circuit. Th e r e st of the pap er is org a niz ed as follow s. The system setup and the prin ciples of the NM - CAP sc h e m e a re de sc r i be d in Sec tion II. Th e r e s ults are d i scus sed i n S ection I II, and fin a lly the con c lusi ons are gi ven in Secti on IV . II. E XP E RIMENTAL S ETU P A s implif ied bl oc k di agr am of t he exp e rim ental s etup for the propos ed NM - CAP sc h e m e is illus trated i n Fig . 1. A pseudo random binary seq uence (PRBS ) D m of l e ngt h 2 15 - 1 i s repe ated for 10 6 bit s for each sub carri er. Th e indiv i du al bits are mapp e d ont o an M -ary quadra t ur e a mp litud e mo dul ation ( M - QAM), w here M i s th e orde r of QAM a nd was se t to 4 in thi s work . The sym bol s e que nces are up- sa mp l ed ac cording to the numb er of s amp les/s ymbol ! " # $ %& ' () * + , - w her e $ ' - is the ceili ng func tion . T he s ignal is th en sp l it into its rea l a nd imagi nary par ts ( i. e. , or in-ph a se ( I ) and quad ratur e ( Q )) and pass e d t hrough th e squ a re r oot ra i s e d cos ine (S RRC ) pul se shapin g f i lt e rs . T he impuls e res ponses o f t he tr a nsmi t fil ter s forms a Hilbe rt pair (i .e., t hey are orthog ona l in the time d omain wi th a 90° phase sh ift) and are giv e n as a prod uct of the SRR C filter i mp ulse respo nse and the c os i ne and sin e w aves for th e I and Q signa l, resp ec t ive ly, which are given resp ecti vely as [12, 15]: . / 0 ( 1 , # 2 34! 5 6 ( ) 7 + ,8 * 9+ 1 : " ;<3 5 6= 8 6 > ) 7 ?9+ 1 : " @ A B C D ;< 3 5 6 ( %& 7 ) , = 8 (1) and . E 0 ( 1 , # 2 34! 5 6 ( ) 7 + ,8 * 9+ 1 : " ;<3 5 6= 8 6 > ) 7 ?9+ 1 : " @ A B C D 34! 5 6 ( %& 7 ) , = 8 (2) where T s is the symbo l dur ation, γ = π t/T s and δ = 1 + β . The fr equen cies of c a rr ie r s, g e ner ated by the pul se shapin g tra nsmi t filt e rs , wi l l be gi ven by : . F G # %! 7 ) %& H ( ) 7 I , (3) where n is t h e s ubcar rier numb er, B i s the total sign al bandw idth and α is defin ed he re as a b andw idth compress ion fa ctor. In th e conve ntional m - CA P sch e m e , α is se t t o 0 , thus maint aining the ort hogon ality betw een subc arri ers. I n the propos ed NM - CAP system , we s hift t he c arrier fre quenci es Fig. 1 The sch emat ic block dia gra m o f the expe ri menta l NM - CA P VL C sys tem . ‘UP ’, ‘ RES ’ and ‘ DOW N’ b lo ck s re f er to up - sampling, resampling and down - sampli ng, respectively towar ds lower va l ues, compress ing by an ov erall factor α , purpos ely overl a ppin g the sub carrie rs and bre a kin g t h eir orthog onali t y. The outpu ts of the filte rs ar e real- valued, and are summ ed to fo rm the discre te tim e domain sign al as g iven by [12]: 3 ( 1 , # J % K ?3 / G ( 1 , L . / G ( 1 , 7 3 E G ( 1 , L . E G ( 1 , @ 0 GMN (4 ) where L r e pres ents the tim e dom ain c onvo l uti on a nd 3 / G ( 1 , a nd 3 E G ( 1 , ar e in phas e and quadra ture compo nents of the M - QAM symbo ls for th e n th sub c arr ier, resp ectiv ely. Not e that t he disc ret e signa l s ( t ) is s ample d a t the s amp ling fre quency giv e n by: . " # O " ! " ) & (5 ) where R s is the sig nal b aud ra te. The gene rated disc rete s igna l s ( t ) is load ed i nto a Rohd e & Schw a rz S M W200A vec tor s igna l gen e r ato r, whi ch i n turn feed s the LED dr i vin g c i r cuit prior to intens ity mo dula tion o f th e l ight sour ce . T he ligh t so urce used wa s a comm e rc ially av ail ab le LE D (OSRAM Gold en Drago n) and the m easur e d fr eque ncy res ponse show ing a 3 dB modu latio n bandw idth of ~1. 2 MHz is i llust rated in Fi g. 2 . Note tha t, we us ed a bia s curren t o f 500 mA to ens ure th e int ensit y mo dulat ion with in th e LED linea r reg i on , see ins et in F ig. 2. T he m odulated l ight was t ransm i tt e d over a ran ge d = 2 m, whi ch is typi cal for indoor V LC lin ks) . At t he Rx, a l ow noise av alanche ph otodet ector (A PD) (Thor labs APD 430A 2/ M) based o ptica l re c eiv e r w ith inbu ilt t rans imped ance ampl i fie r wa s used t o reg ener ate th e elec trical sign al. Two 25.4 mm biconv e x lenses were plac e d at dist a nc es of l 1 = 25 mm and l 2 = 35 mm from the L ED and the PD , respe ctiv ely, to foc us and c ol li m ate th e e mi tted and rec eiv e d light be a m s . The reg ener at ed si g na l was captur ed using a LeCroy W a veRu nner Z640 i rea l time oscil l os cop e with a s a mp ling rate of 50 MS /s for fu rthe r offl ine proc essing in MA TL AB . Th e si gnal p rocess ing a dopted to re cover the dat a inform ation is the sam e as in the trad i tio nal m - CAP sch eme [1 5] . Follow ing r esam plin g (d enoted ‘RES’ i n Fi g. 1), the sign al is pass e d thr ough the tim e-revers ed fi lters m a tch e d t o t he tr ansmit filt ers as P / 0 ( 1 , # . / 0 (71, and QP E 0 ( 1 , # . E 0 (71, for the r ea l and imag i nar y parts of the signal , re s pectiv ely , see Fig. 1 . T he signa l i s down-s ample d and d emo dula ted to recove r th e rece ived M -QAM sym bols for B ER estim at ion . T he res ul ti ng BER perform anc e is determi ned by compa ring the trans mi t ted and recei ved b its i n a bit-by- bit m anner , a nd w e set t he BER floo r to 10 -4 . III. R ESULT S AND D IS CUS S ION In this s ectio n w e pres ent re s ult s for the BER perf orm ance per sub carri er as a func tion of the sub carr ier inde x n for m = 10 and 2, for b = 0.1, 0.3 and 0 . 5, and α = 0.1, 0 . 2, 0.3 , 0.4 and 0. 5 . For m = 10 an d b = 0.1 as sh own in Fig. 3 (a ), it i s evid ent tha t a comp re ss i o n rat io ³ 20% (i. e. , α ≥ 0.2) c ann ot b e suppor ted using the 10- CA P at a B ER le vel be lo w t he 7% forw ard error corre ction (F EC) li mit of 3. 8 × 10 -3 (show n as a dash ed lin e i n Figs . 3 and 4) . This m eans t hat t he data c an be com press ed by 10% wit h a BER low er tha n 10 -4 for ever y s ub car rie r. It shou l d be not ed t hat th e α = 0.1 cur ve i s not show n here , s i n ce no err ors were found in the ent i re sym bo l stream . T hus , shou ld the BE R resul ts for any value of α be lowe r than the 10 -4 BER floor , the curv e wil l be omitted fr om the figure . Inter estingl y, the BER valu e s for the compr ession ratios of 20% and 30% are low e r f or n = 1 and 10 . The re ason for this is du e to the fact t hat each of th e s ubc arri ers only in t erf e r es with its adjac e nt sub carri e r, i.e., they only ha ve one inte rfer ing sub carri e r. How ever, for each subca rrier ind ex n = 2 – 9 , ever y ind i vidu al su bcarrier in ter fer e s with its two adj acent subcar riers , t hus r esultin g in inc reas ed BER. A lso show n in Fig. 3(a ) are t he cons tella tion inte nsity maps for n = 1, 2 and 1 0 and for α = 0. 3 . C ons ider i ng 4- QA M with a c omp ressio n o f 10% , and b = 0.1, t he resu lting spectr al effi c ien cy is 2. 02 b/ s /Hz , w hich hig her t h an 1.82 b/s /H z for the uncom press e d cas e with no d e gra dation in the BE R perform anc e , t hus g i v ing a net spec t ra l effi c ien cy ga i n of around 11%. In F ig. 3( b), f or b = 0.3 and m = 10 , i t is possib l e to r ecover the signa l with a BER < 10 -4 for α = 0. 2 and henc e, the d a ta is not plot ted bec ause ever y sub carrier i s error fre e, a nd on l y plo ts for α > 0.3 are sh own. T he reason for the i m provem ent in the α = 0. 2 ca s e over the prev ious re sul t s is due to the add ition al exc e ss ba ndwid t h i ntrod uced by the sys tem in terms of b . S ince incr easing b m eans the fil t er ro ll-of f is slow er and hen ce, the bandw idth is wide ne d . There fore, a t th e b and- e dg es, th e sys tem can cope with a ddi tional interf e ren ce due to a low e r leve l of in ter -mo dulat ion betwe e n subc arri ers. Hen ce, a spectral eff ici enc y of 1 .9 2 b/ s /Hz ca n be obt ained over th e ori ginal spect ral effi ciency of 1 .54 b/s /Hz , giv i n g a net spect ral effi c ien cy gain of 25% . Inter estingl y, t his g ain in sp ectr al eff ici enc y is no t equ al to th e 20% com pressi on ra te and actu a lly exc e eds it , w hich m ay be diffe r fr om assum ptions . The mai n reaso n s tems from the fa ct the bandw idth B is fi xe d a t 3 MH z and th e bi t ra t e is giv en by O R # S

0.3 t h e BER leve l s have i ncreas e d well b e y ond the FE C limi t. Also s how n are the cons tellat ions for n = 4 and 6 fo r α = 0.3, a n d n = 9 for α = 0.4, res pect ively. Not e th at, t he sp e ctr al effi ciency i s now incr eas ed to 1.91 b/s/ Hz for α = 0.3 , com par ed to 1.33 b/ s/ Hz for the unc ompress ed case , thus re prese nting an impr oveme nt of 44% , w hic h is the h igh est relat ive impro vemen t rep ort ed in this work . On the oth er hand, despi te the fact t hat th e larges t c ompress ion fac t or is used , the spect ral effi cienc y reco rded for α = 0. 3, m = 10 and b = 0. 5, repres ents t he sm al l est sp ectr al effi ciency of the th ree cas es demo nstrat e d so far , by appro xi m ately 0.11 b/ s / H z . How ever, such a sma ll diff erence in spectr al effici enc y may b e a cost w orth payin g, sin ce CAP systems with high er va lues of b are mo re tolera nt t o timing jitt e r due to t he w i der hori z ont al o peni ng of th e p ulse. This is adva ntag eous, gener ally, but is espe cially relev ant whe n c ons ider i ng tha t the re ceive d eye open i n gs red uce with incr e asin g carr ier fre quency [11] . H owe ver, this requir e s furth er inv esti gation, wh ich w ill b e perform e d i n fu t ure w ork. Fig. 3 BER pl ot s fo r (a) m = 10 a nd b = 0.1, (b) m = 10 an d b = 0. 3 an d (c ) m = 10 and b = 0. 5 A s imilar trend is obser ved fo r m = 2 and b = 0.1 , as illus t ra t ed in Fig. 4(a) , w hich show s the BER p erform ance of the two -s ubc arrier sys tem. Fig. 4 BER p lots for (a) m = 2 and b = 0. 1, (b ) m = 2 and b = 0. 3 and (c) m = 2 and b = 0. 5 It is cle a r th at a lin k wi t h α = 0 .1 ca n sup port a BER exact ly equa l to the FEC t hresho ld of 3.8 ´ 10 -3 . Howe ver, for α > 0.1 the BER pe rforma nce is incr eased bey ond th e FEC lim it . T h is means that for m = 2 the m aximum s pectra l ef ficien cy avai l ab le is e xac t ly the sam e as th e cas e for b = 0. 1, and m = 10, i.e. a spect ral eff i ci e ncy of 2 . 02 b/s/H z, repres entin g a gain of 11 % over the non- c omp ressed ca s e, alb eit a t a h ighe r error ra t e ( not e th at in th e previ ous c a s e th e BER wa s < 1 0 -4 ). T hi s i s a n impo rtant resu lt, becaus e on e mus t also consi der t he comp utation al com plexi t y wh e n d esign i n g CAP sys tem s , gene rally. T he 2 -CAP system us es 8 F IR fil ters (t wo pairs for in - phase /quadr ature at bot h the transm it ter and receiv er, for two chan nels) wh ile t he 10-CA P s ystem uses 40 fi lte r s , which repres ents a 5 00% increas e in the c om parat i ve compu tatio nal comp lex it y . Als o dep icted i n Fig. 4(a) ar e two co nstell ation intens ity m a ps for both s ubcarr i ers and α = 0. 1. Fig. 5 Th e r ece ive d elec trica l sp ec tra of th e 2 - CA P s ig nal sh owing t h e att enuatio n caused the l ow modulat ion bandwidth of the LED f or α = 0 and b = 0. 1 In Fig. 4 (b) show BE R pe rf or ma nce f or m = 2 , b = 0.3 and the sam e com press ion rat io as in Fig. 4(a) . N ot e that , in this ca se it is possi ble t o supp ort α = 0 .2 . Th is is due to the w ider subca rrier band width, wh ich has in cr ea s ed 5 - fo ld to 1.5 M Hz/subc arrie r in comp arison t o m = 10 . How ever, wi d e r bandw idths are more susc e pti ble to the freq uen cy attenu ation expe rienced by th e LED’s low mo dulat i on ba ndw idth , whi ch is illus t ra t ed using t h e el ectric al sp ectrum of t he re c eiv e d s ign al in Fig. 5 ( α = 0 , b = 0.1 ). Not e, the incr eased attenua tion for the secon d sub carrier . Hen ce, no add itional spe ctr al effi cien cy gai n can be su pport e d in spi te of t he hig her ro ll-off fa c tor , whi c h i s a disadv antag e of a low er numb e r of su bcarriers . Const ellati ons are show n ins e t for both α = 0 . 2 su bcarr iers (b ottom ) and α = 0.3, n = 1. In teres t ing ly, th e c ons t el lations sh ow th e inter feren c e patter ns du e to in termodu lation of the subc arri ers a nd thei r determ inisti c nature . S inc e the rec e ive d symb ols are in fixe d posit ions, th e i nterm i xing of subc arri e rs caus es the comp lex multip lica tion of the two expe c ted cons tell a tio ns, resul ting in new re ceived const e ll a tio ns wi th the obs erved int erfe ren ce patter ns. For t he fi na l ca se w ith m = 2, b = 0.5 , a nd α = 0. 3 - 0. 5 the BER p erform a n ce is illust rat ed in Fi g. 4 (c ) . The b est B ER perform anc e (i .e., low er th a n t he FEC limi t of 3. 8 × 10 -3 ) i s achie ved for α = 0.2 for m = 2 i n Fig . 4(a) . Note th a t, t he s pec t ra l eff ici enc y is also le ss than m = 10 , thus s uppor ting a tot a l spect ral e ffi ciency of 1.67 b/ s / H z comp a r ed to 1 . 33 b/s/H z for the uncom pressed cas e, bu t a t a much lower compu ta ti onal comp lexity. Since no addition al fun ction a li t y is a d ded at the rec e iv er or transm itter in c om paris on to t ra dition al orthogo nal m - CAP , a ll of th e resu lts present ed he re dem onstra te impr oved spectral effi c ien cy, wi t h a m aximum gai n of 44% ( m = 10, b = 0. 5) w it h no add ition al compu tationa l compl e xit y in compa ri s on to trad i tio nal 10- CAP sys t em wit h no BER pe rform ance degr adation IV. C ONCLUSIONS In this pap e r, w e propos e and experim ental l y ver ify a nov el modu lation sch eme calle d NM - CAP for inc reas i ng the s pec tra l effi c ien cy in band l im i ted VL C s ystem s . We demo nstra ted th at for m = 10 , ban dwi dth compress ion up to 30% (i.e . 44 % impr ovement i n th e s pectr a l e f fic ien c y) can be sup port ed witho ut an y B ER p erform ance d egrada t ion w hen comp a red to a conv ention a l m - CA P . We a lso show t hat h igher mod ulati on order sys te ms can su ppo rt hig her ba ndw i dth s avings tha n lower order on e s. Moreo ver, the propos ed novel sch eme does not intro duce addi tional c om putat i ona l com plex ity , whi ch is a signif icant a dvanta ge. In futur e work we will focus on the optimi zati on of NM - CAP sys tem b a s e d on t he me a su red s i gn al- to - noise r atio val ue s for indivi dual sub carri e rs and bit- a nd pow e r-lo a d ing alg orit hms. A CKNOWLEDG E MEN T This w ork was suppo rted by the UK E PSR C g rant , EP/P 00628 0/1: Mul tifun ction al Po lymer Lig ht-Em ittin g Diod e s with V i sibl e L ight Com muni c at i ons (MA RV EL) a nd by GA CR 17-175 38S. R EFEREN CE S [1] Z. 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